1. Field of Invention
The present invention relates to a sound transducer and more particularly to a substantially flat and circular configured hydrophone having directional and omnidirectional response characteristics and which is especially suited for use in applications requiring a directional hydrophone capable of being flush mounted in a surrounding surface.
2. Description of the Prior Art
Hydrophones having directional characteristics for use in determining direction of propagation of incident sound waves are well-known in the art. These prior art devices range from relatively large directional arrays of two or more spaced hydrophones to directional hydrophones housed in a single and relatively small package.
In the conventional prior art spaced array, identical but separate pressure sensitive omnidirectional hydrophone transducers are placed apart in the water transmission medium and electrically connected so as to provide the desired directional characteristics. Accuracy of these arrays depends to a large extent upon a close match of the individual hydrophone sensitivities under various environmental conditions such as, for example, temperature and static pressure. These arrays normally measure the acoustic pressure differential between the spaced hydrophone positions, and the signal outputs of the individual hydrophone transducers are combined to provide output signals bearing a sine and/or cosine like function of the angle of incidence of the incident sound waves, as is well-known to those skilled in the art. The combined signal output is a function of transducer spacing as well as the angle of incidence. Ideally these arrays use hydrophone spacings of one-eighth wavelength or less of the sound wave of interest in the sound transmission medium in order to provide a differential combined output signal having a true sine or cosine response pattern. This requirement thus tends to restrict the frequency range over which a given configured array will provide true response patterns and yet provide adequate signal output levels. Lower operational frequencies generally require arrays of larger physical configurations.
Most shipboard or sonobuoy applications require directional hydrophones configured in a single relatively small package. Many prior art hydrophones meet this requirement. Typical of these latter-mentioned devices are the cylindrical, reed, and spherical-type directional hydrophones, examples of which are respectively disclosed in U.S. Pat. No. 3,496,527 issued to Ziehm and entitled "Transducer for Determining the Angle of Incidence of Sound Waves"; U.S. Pat. No. 3,603,921 issued to Dreisbach and entitled "Sound Transducer"; and U.S. Pat. No. 3,732,535 issued to Ehrlich and entitled "Spherical Acoustic Transducer".
Although the latter-referenced hydrohones, as well as other similar prior art devices, are capable of satisfactory operation and are suitable for some shipboard and sonobuoy applications, they are not ideally suited for use in applications where it is desirable that the hydrophone be flush mounted in a surrounding surface such as the hull of a vessel. The inherent nature of these prior art hydrophones requires that the acoustical sensitive or active surfaces of the hydrophone be surrounded by the water transmission medium. Placing such type hydrophones within a well or recess in a surrounding mounting surface affects the effective coupling betwween the hydrophone's active surfaces and the main of the acoustic transmission medium and, in addition, can create a discontinuity of the transmission medium surrounding the hydrophone as well as the creation of multiple sound transmission paths in the transmission medium within the recess, all of which can result in degradation of the hydrophone performance. Use of these same prior art hydrophones extending beyond the surrounding mounting surface is also disadvantageous since such protrusion not only adversely affects the hydrodynamic characteristics of the outside surface of the hull but also makes the protruding hydrophone highly susceptible to physical damage. In addition, the ambient noise output levels of these prior art hydrophones when so mounted are usually undesirably high due to the flow of water about the protruding hydrophone structure. It is thus apparent that such prior art directional hydrophone devices are not ideally suited for certain seaborne applications.